Constant ON-time control is widely used in applications of switching converter because of its excellent load transient response, simple circuitry structure and smooth operation mode switching. In most applications, a comparator is used for comparing a feedback signal reflecting an output voltage of a switching converter with a reference signal. Based on a comparison result, the switching converter controls at least one switch to adjust the output voltage. For example, when the feedback signal is less than the reference signal, the at least one switch is turned ON until a predetermined ON-time period expires.
However, if an output voltage ripple is large, the comparator actually compares valley of the feedback signal with the reference signal in each working period. There exists an error between the actual output voltage and a set target of the output voltage. For applications with small output voltage, such as DSP/CPU/Memory power supplies, the error generated by the output voltage ripple will seriously affect output voltage accuracy of the switching converter. For example, if the set target of the output voltage is 1V and amplitude of the output voltage ripple is 50 mV, then the actual output voltage is 2.5% higher than the set target of the output voltage.
One of the present solutions is to introduce an error compensation signal by an error amplifier based on a difference between the output voltage and the set target of the output voltage. For example, the error compensation signal is subtracted from the reference signal as a corrected reference signal which is compared with the feedback signal. However, it is not easy to design the error amplifier. On one hand, fast response of the error amplifier will lower transient response of the switching converter. On the other hand, slow response of the error amplifier will prolong the time period that the output voltage achieves the set target.